Furnaces – Refuse incinerator – For liquid refuse
Patent
1996-12-17
1998-06-30
Bennett, Henry A.
Furnaces
Refuse incinerator
For liquid refuse
110243, 110244, 110245, 110251, F23G 704
Patent
active
057718172
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to recovery boilers for combusting spent liquor from cellulose cooking while at the same time recovering chemicals. Apart from water, spent liquor of this nature, which is often termed black liquor, also contains organic residues, principally lignin residues which have arisen during the cooking, and inorganic chemicals, principally sodium sulphate which is used in the cooking. The purpose of the combustion is, in addition to the necessity of disposing of the spent liquor in an environmentally friendly manner, that of extracting heat by means of the combustion, and thus making the whole process more economical, and also that of recovering the inorganic chemicals so that they can be reused, something which additionally reinforces the economy of the process and contributes to a decrease in environmentally inimical discharges.
STATE OF THE ART
Recovery boilers for combusting spent liquors and recovering chemicals during the manufacture of cellulose have been known for decades. While there are a number of different constructions, the general principle is based on the recovery boiler, which consists of an upright cylindrical or polygonal furnace which is clad with water pipes on the inner side, being supplied with spent liquor through nozzles, at a certain level, and with so-called primary air and secondary air, below this level, and with tertiary air, and, where appropriate, quaternary air, at one or more levels above that for the injection of the liquor. In the furnace, in which the temperature is in the region of 1000.degree. C., the water in the liquor drops, which are finely dispersed, is firstly vaporized, after which the organic material is pyrolyzed and converted into combustible gases and carbon, which latter, together with the inorganic chemicals in smelt form, is separated off from the gas by falling down to the buffer bed or else first being slung against the walls and then running down to the bottom, from which the smelt is removed. While there is a reducing atmosphere in the lower part of the furnace, so that sodium sulphate (Na2SO4), for example, is reduced to sodium sulphide (Na2S), the atmosphere in the upper part is oxidizing to ensure that the organic compounds are combusted to completion and that the temperature is increased. While the reducing atmosphere, which is due to an under-stoichiometric supply of oxygen, is intended to reduce the chemicals, it also acts as a brake on the formation of nitrogen oxides, which are harmful to the environment.
That which determines the speed of drying, pyrolysis and combustion of the coke residue is the transfer of matter or heat between a gas and a solid or liquid particle, in which transfer at least two phases thus participate. More recently, it has been found that, in order to create favourable conditions for the exchange of matter and heat, the shearing velocity and the mixing between gas and particles should be increased, and, as a consequence, the air/gas mixture has been arranged to rotate. This results in a more uniform gas velocity, improved drop separation, a higher combustion temperature, a higher degree of reduction and a lower requirement for air in the lower part of the boiler furnace. The lower requirement for air in the lower part of the furnace of the recovery boiler can be utilized, inter alia, to move a larger part of the combustion higher up the furnace and thereby achieve lower emissions of nitrogen oxides. This rotational procedure, which is termed "rotafire", is described in more detail in European Patent Application No. 92915760.0.
Technical Problem
The principle of a rotating gas mixture in the furnace creates substantially improved conditions for the course of the reaction in the furnace. The advantages achieved are, however, limited in ordinary recovery boilers since there is a limit to the extent to which the speed of rotation can be increased. Thus, if this speed exceeds a certain level, a downwardly directed vortex arises in the centre of the furnace, resulting in an undesir
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Bergman Jan
Jansson Sven-Erik
Larsson Agneta
Olausson Lars
Bennett Henry A.
Fasth Rolf
Kvaerner Pulping AB
Lu Jiping
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